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Shoulder muscle fatigue development in young and older female adults during a repetitive manual task ab

b

a

Jin Qin , Jia-Hua Lin , Bryan Buchholz & Xu Xu a

b

Department of Work Environment, University of Massachusetts, Lowell, MA 01854, USA

b

Liberty Mutual Research Institute for Safety, Hopkinton, MA 01748, USA Published online: 06 May 2014.

To cite this article: Jin Qin, Jia-Hua Lin, Bryan Buchholz & Xu Xu (2014) Shoulder muscle fatigue development in young and older female adults during a repetitive manual task, Ergonomics, 57:8, 1201-1212, DOI: 10.1080/00140139.2014.914576 To link to this article: http://dx.doi.org/10.1080/00140139.2014.914576

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Ergonomics, 2014 Vol. 57, No. 8, 1201–1212, http://dx.doi.org/10.1080/00140139.2014.914576

Shoulder muscle fatigue development in young and older female adults during a repetitive manual task Jin Qina,b*, Jia-Hua Linb, Bryan Buchholza and Xu Xub a

Department of Work Environment, University of Massachusetts, Lowell, MA 01854, USA; bLiberty Mutual Research Institute for Safety, Hopkinton, MA 01748, USA

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(Received 3 July 2013; accepted 31 March 2014) Age may modify the association between occupational physical demand and muscle loading, and ultimately increase the risk of musculoskeletal disorders. The goal of this study was to investigate age-related differences in shoulder muscle fatigue development during a repetitive manual task. Twenty participants in two age groups completed an 80-minute simulated lowintensity assembly task. Electromyographic (EMG) manifestation of muscle fatigue was observed in the upper trapezius, deltoid and infraspinatus muscles in both age groups, and coincided with an increase in the subjective ratings of perceived exertions. Compared with the younger group, older group showed a more monotonic decrease in EMG power frequency in the upper trapezius and deltoid muscles. However, the age-related difference in EMG amplitude was less consistent. Relative rest time of the upper trapezius muscle in the older group was less than the young group throughout the task. The observed patterns of EMG measures suggest that older participants may have disadvantages in fatigue resistance in the upper trapezius and posterior deltoid muscles during the simulated repetitive manual task. Practitioner Summary: A rapidly ageing workforce in the USA and other countries poses new challenges for preventing work-related injuries. This study showed that during an 80-minute repetitive light manual work, older adults exhibited more consistent patterns of electromyographic manifestation of shoulder muscle fatigue and less rest in the upper trapezius muscle than young adults. Keywords: age; shoulder; muscle fatigue; electromyography; repetitive work

1.

Introduction

Shoulder disorders and complaints constitute an important health problem in the working populations. In 2005, shoulder disorders amounted to 77,800 or 6.3% of all non-fatal injuries in private industry and 39% of all work-related musculoskeletal disorders (WMSDs) in the USA (BLS 2006). The prevalence of shoulder pain reported in the general population is 6 –11% under the age of 50 years, increasing to 16 –25% in population of 56 years and older (Badley and Tennant 1992; Bjelle 1989). Occupations that are most affected include computer users and workers on assembly lines, and in construction, food/meat processing, textile and packaging industries (Sommerich, Mcglothlin, and Marras 1993; Leclerc et al. 2004). Many of the occupations with high prevalence of shoulder disorders involve repetitive light manual tasks in terms of external load handled. Repetitive arm movement is one of the main occupational risk factors for shoulder disorders and complaints (Larsson, Sogaard, and Rosendal2007; Nordander et al. 2009; Van Rijn et al. 2010). Repetitive arm movements at low force levels [less than 20% of the maximum voluntary contraction (MVC)] common in working life can result in fatigue in the trapezius muscle (Bosch et al. 2009; De Looze, Bosch, and Van Dieen 2009). Muscle fatigue, often defined as a transient decrease in the capacity to perform physical actions (Enoka and Duchateau 2008), has important implications for WMSD risks. Although the exact mechanisms and dose – response relationship in low-intensity tasks were not well understood, muscle fatigue when measured during work could be a relevant biomarker for cumulative exposure at work (Dennerlein et al. 2003), and may serve as a surrogate indicator of risk (Nussbaum 2001). Moreover, muscle fatigue has been hypothesised to be a precursor of shoulder complaints (Rempel, Harrison, and Barnhart 1992; Takala 2002). The number of workers aged 55 years and over is projected to grow from 13% in 2000 to 25% in 2020 in the USA (Toossi 2012). Age has been associated with increased risk of WMSDs (Roquelaure et al. 2009). People aged 55 years and over were found to have a greater risk of diagnosed chronic shoulder disorders than younger people (Miranda et al. 2008). A review of the literature suggests that age effect on muscle fatigue may depend on contraction mode (isometric and dynamic) (Kent-Braun 2009). Older individuals have been reported to be more resistant to muscle fatigue than younger individuals in isometric contractions. For example, older adults exhibited longer endurance time and slower development of

*Corresponding author. Email: [email protected] q 2014 Taylor & Francis

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local fatigue than young adults during submaximal isometric contraction with the elbow flexor muscles and shoulder abduction muscles (Hunter, Critchlow, and Enoka 2005; Yassierli et al. 2007). However, this age difference was not observed during dynamic contractions, i.e. Callahan, Foulis and Kent-Braun (2009) found that fatigue resistance in the knee extensor muscles was similar among young and older adults during dynamic contractions. Others reported that older adults develop greater fatigue during dynamic contractions (Baudry et al. 2007; Dalton et al. 2010). In previous studies investigating the association between age and muscle fatigue, the protocols to fatigue muscles were mostly at relatively higher intensity levels or at maximum capacity. Such experimental conditions may not represent the physical exposure during repetitive light manual work in many occupational settings. In addition, muscle fatigue parameters were often measured at baseline and the end of the fatigue protocol. The temporal patterns of muscle fatigue development are also of interest for ergonomic assessment and intervention. Studies of muscle fatigue development during occupational tasks, particularly those involving dynamic motions, could provide valuable information about relations between workplace risk factors, muscle fatigue and discomfort. Despite the epidemiological evidence relating age and MSDs in the shoulder region, little is known about how age modifies local muscle fatigue during light manual tasks. The purpose of this study was to determine shoulder muscle loading as well as its temporal patterns among young and older female adults during an 80-minute low-intensity repetitive manual task, which was designed to simulate light assembly tasks in occupational settings. Surface electromyography was measured to show manifestation of muscle fatigue. Myoelectric manifestations of muscle fatigue include an increase of electromyographic (EMG) amplitude and a decrease of its power spectral frequencies (Basmajian and De Luca 1985). It was hypothesised that older adults would be more fatigable than young adults during the simulated task. 2.

Methods

2.1 Participants Twenty female participants with no history of or current upper extremity musculoskeletal complaints were recruited from local communities into two age groups (ten in each). The average ages of the young and older group were 25.2 (SD ¼ 3.9, range 20– 32) and 61.7 (SD ¼ 4.3, range 57– 68) years, respectively. The anthropometric measurements were similar between the two groups (Table 1). None of the participants had prior experience in industrial assembly type tasks. All participants were right-handed except two in the younger group and one from the older group. Each participant provided informed consent, and the Institutional Review Board approved the study protocol. The study participants only include female adults because women have been reported to have higher risk of work-related upper extremity disorders than men (De Zwart, Frings-Dresen, and Kilbom 2001; Silverstein et al. 2009). Table 1. Participant age and anthropometry mean values (and standard deviation) across subjects within the age groups, and p-values of Wilcoxon Kruskal – Wallis two-sample tests between young and older groups.

Age (year) Height (cm) Weight (kg) Circumference (cm) Upper arm Forearm Hand Length (cm) Upper arm Forearm Hand Estimated mass (kg)a Upper arm Forearm Hand Baseline EMG RMS (mV) Upper trapezius Anterior deltoid Middle deltoid Posterior deltoid Infraspinatus a

Young (n ¼ 10)

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p

25.2 (3.9) 164.8 (5.2) 64.1 (10.9)

61.7 (4.3) 165.8 (9.8) 72.9 (17.8)

,0.001 0.820 0.198

29.3 (5.1) 21.2 (1.9) 18.4 (1.4)

31.5 (5.8) 22.6 (3.0) 18.9 (1.1)

0.364 0.130 0.383

30.8 (2.8) 24.8 (1.1) 17.4 (0.8)

30.3 (2.4) 24.9 (2.2) 17.8 (1.3)

0.940 0.595 0.762

1.84 (0.61) 0.77 (0.20) 0.37 (0.07)

1.69 (0.51) 0.77 (0.15) 0.37 (0.06)

0.545 0.597 0.880

263.4 (103.8) 162.9 (69.9) 159.9 (59.4) 127.4 (54.9) 162.8 (112.0)

253.6 (139.7) 219.2 (132.1) 187.1 (104.4) 127.8 (91.8) 96.7 (62.5)

0.597 0.364 0.545 0.597 0.151

The arm masses were estimated based on segment length and circumference (Zatsiorsky 2002).

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2.2 Materials The design of the task was to simulate light assembly work in which the hand moves in a functional space on a workstation. The experimental task required participants to reach and pick up colour-coded washers from container bins and stack them on vertical dowels (Figure 1(a)) using their dominant hand. Both the container and the dowels were colour-matched with the washers, and the sequence of colours was the same for the five bins and columns of dowels. The outer and inner diameters of the steel flat washers were 30 and 18 mm, respectively. The size of the container bins measured 130 (L) £ 105 (W) £ 80 (H) mm. The assembly piece was a 195 (L) £ 120 (W) £ 18 (H) mm wooden baseboard with 15 plastic dowels in three rows and five columns. The cross-sectional diameter and height of the dowels were 10 and 65 mm, respectively. The distance between adjacent dowels was 40 mm. The baseboard was placed 15 cm away from the edge of the workstation. A centre point was marked at the edge of the workstation to be aligned with the centre line of the longer side of the assembly piece baseboard. From the centre point, five straight lines were drawn like wheel spokes on the workstation surface in 258-angle intervals (Figure 1(a)). For right-handed participants, the first line from the right was 258 from the edge of the workstation, and vice versa for the left-handed participants. The five bins were positioned on these five lines at distances adjusted for each individual’s arm length (see Section 2.3). Such set-up was designed to represent a functional space of arm movement during occupational tasks (Choi and Mark 2004; Mark et al. 1997). Three muscles in the dominant arm were monitored using surface electromyography (EMG): the descending part of trapezius, deltoid (anterior, middle and posterior) and infraspinatus. These muscles were chosen because they play import functional roles in reaching tasks, are common sites for MSD conditions (e.g. trapezius myalgia, rotator cuff syndrome) and can be monitored by surface EMG. After skin preparation (gentle exfoliation and cleansing with an alcohol pad), the myoelectric activity was recorded telemetrically by means of bipolar surface electrodes (Ag-AgCl, Noraxon USA, Inc., Scottsdale, AZ, USA) with an inter-electrode distance of 20 mm. The electrode placement locations followed recommendations from the SENIAM Project (Hermens et al. 2000). A reference electrode was placed on the acromion process. Data were acquired on a computer with a 16-bit analogue-to-digital converter (National Instruments, Austin, TX, USA) at a sampling rate of 1024 Hz. In offline data processing, the EMG signals were digitally band-pass (10 –350 Hz) filtered with a fourth-order bidirectional Butterworth filter.

Figure 1. (Colour online) (a) Experiment set-up. Letters indicate the colour of washers (and corresponding bins) in black (BL), white (WH), blue (BL), yellow (YE) and red (RE). (b) Experimental procedures.

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2.3 Experimental protocol Participants performed the task while seated; the workstation and chair heights were adjusted so that the participant’s knee angle was about 908 and the seated elbow height was the same as the workstation height. Participant’s shoulder on the dominant side was aligned with the centre point of the workstation, with the anterior edge of their torso approximately 100 mm away from the edge. The five bins were placed along the five lines at a distance whereby the fingertip of the fully extended arm touched the back of the bins. Participants were instructed to pick up one washer at a time and place the hole over the dowel of the same colour, then to progress from left to right (from right to left for left-handed participants). Participants went through all five dowels in one row and continued to the next row until all three rows were completed. This process was repeated for a total of 80 minutes with short breaks after every 20 minutes. After every 10 minutes, the assembly piece was fully loaded, and it was replaced by an empty piece. Each task cycle consisted of reaching, grasping a washer, moving back, aiming and releasing the washer. Participants were asked to complete each task cycle within 2 seconds following a metronome at 0.5 Hz. This speed was determined based on the methods-time measurement (MTM) technique (Niebel 1988). Cycles during work were paced with a cycle time corresponding to 120 MTM. This protocol was designed to simulate a ‘line-type’ production with little autonomy as to work pace. None of the participants had difficulty performing the task at this pace. Participants were instructed to ignore any errors made (e.g. drop a washer) and proceed to the next one. Prior to data collection, participants practiced the task for 2 minutes or until they felt familiar with the task. A participant was considered ready if she was able to perform the task as instructed for at least 1 minute during the training. Since this task was designed to be monotonous and simple, all the participants were able to fulfil this criterion within 2 minutes. Participants completed the task while keeping the trunk in an upright posture. An investigator monitored the participant throughout the task to make sure that she maintained proper trunk posture and speed. Isometric tests and ratings of perceived exertion were performed at baseline and every 20 minutes during short breaks of 2 minutes (Figure 1(b)). During isometric tests, participants sat with shoulder abducted at 908 in the scapular plane (approximately 458 internally rotated) in the following two postures (Boettcher, Ginn, and Cathers 2008): (1) elbow extended, and forearm pronated like the posture of emptying a can and (2) elbow in 908 flexion and thumb pointed towards their body. A 1.36 kg external load was suspended from the distal end of the forearm. Participants held the postures for 15 seconds and 10 seconds of data (5 – 14 seconds) was used for data analysis. The EMG amplitude measured at baseline during isometric tests was also used as reference voluntary exertion (RVE) for normalisation. Ratings of perceived exertion (RPE) were recorded using Borg CR-10 scale (Borg 1982). The investigator explained the rating scales, the shoulder area (that includes all muscles under investigation), and asked the participants to rate how hard they felt their body was working in the shoulder region.

2.4

Data analysis

The dependent variables included EMG root mean squared (RMS) amplitude, median power frequency (MPF) and muscle relative rest time (RRT). RMS amplitude was calculated with overlapping moving windows of 100 ms (Mathiassen, Winkel, and Ha¨gg 1995) and normalised to RVE. MPF was computed with a moving window of 256 ms using a short-time fast Fourier transformation (Hostens and Ramon 2005). RRT was defined as the duration for which muscle RMS amplitude was lower than 10% of RVE and was calculated as a percentage of the total recording time. This threshold was based on preliminary testing (10% RVE < 1.8% MVC in the upper trapezius muscle) and values reported in the literature (Hansson et al. 2000). RRT indicated the total time of short unconscious interruptions in EMG activity – gaps, which were shown to ˚ stro¨m 1997; Sandsjo¨ et al. 2000; Veiersted, be correlated with future WMSD symptoms and complaints (Ha¨gg and A Westgaard, and Andersen 1993). During each 20-minute session (Figure 1(b)), six 1-minute data samples were extracted with 2-minute intervals. A total of 24 (6 £ 4) samples during the 80-minute task were used for data analysis. For each data sample, mean (SD) values for the dependent variables were calculated. The independent variables include age (young and older), sessions (1 –4) and samples within a session (1 – 6), as well as age interactions with session and sample. Two time domain variables (session and sample) were used for statistical analysis because it is of our interest to determine the temporal trend both across and within sessions. For RPE and EMG measurements during isometric tests, the main effect of time (0, 20, 40, 60 and 80 minutes) and age and their interaction were the independent variables. Mixed effect model (PROC MIXED procedure in SAS, Cary, NC, USA) was used to evaluate age, session, sample (nested within session) and relevant interactions on muscle EMG measures and RPE. Tukey’s post hoc analyses were performed to test for differences among pairs when appropriate. The level of significance was chosen at p , 0.05.

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3. Results 3.1 EMG during dynamic contractions 3.1.1

Time-related changes in EMG signals

The RMS amplitude of the upper trapezius muscle increased and the MPF decreased over time both across and within sessions ( p # 0.011) (Figure 2; Table 2). The RMS amplitude of the anterior, middle and posterior parts of the deltoid muscle increased and the MPF decreased over time across sessions ( p , 0.0001) (Figure 2; Table 2). Post hoc analyses indicated that in both age groups the EMG amplitude of the deltoid muscle increased within the first and second sessions, but did not change afterwards. In addition, the MPF of the anterior and middle deltoid muscles showed decreasing trend within sessions ( p # 0.004). For the anterior and middle parts of deltoid muscle, RRT increased across sessions ( p # 0.001) (Figure 3; Table 2). The RMS amplitude of the infraspinatus muscle increased and the MPF decreased across sessions ( p , 0.0001). RRT of the infraspinatus was not affected by the independent variables.

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3.1.2

Age-related differences in EMG signals

Main effect of age was found only for the upper trapezius muscle. MPF in the older group was higher than the young ( p ¼ 0.035). The EMG amplitude of the upper trapezius muscle tended to be higher in the older group than the young ( p ¼ 0.074). RRT among the older group was on average 2.7 (1.3)% less than the younger group ( p ¼ 0.048) (Figure 3; Table 2). 3.1.3

Interactions between age and time-related changes

There were interactions between age and session in EMG amplitude for the middle and posterior deltoid muscles ( p # 0.001), which was due to the faster increase in the first session among the older participants levelling off the overall trend across sessions (Figure 2). The decrease of MPF in the older group was faster than the younger group for the upper trapezius and deltoid (anterior, middle and posterior parts) muscles as suggested by the age £ session interactions ( p # 0.001) and post hoc analyses (Figure 2; Table 2). Deltoid muscle MPF decreased monotonically over time in the older group, but only decreased in the last session for the younger group. No age £ sample interaction was observed and, therefore, not presented. In addition, there was no interaction effect observed in RRT. 3.2 EMG during isometric contractions The RMS amplitude measured during isometric tests increased 42 (34)%, 32 (29)%, 35 (32)%, 30 (39)% and 31 (30)% RVE from baseline to the end of 80-minute task for the upper trapezius, anterior, middle, posterior deltoid and infraspinatus muscles, respectively ( p # 0.001) (Table 3). MPF of the upper trapezius, anterior and middle parts of the deltoid muscle decreased over time compared with the baseline, but not for the posterior deltoid and infraspinatus muscles. No age main effect or interaction was observed for isometric EMG measures. 3.3

Ratings of perceived exertion

RPE in the shoulder region increased from zero (‘nothing at all’) at baseline to 4.0 (SD 2.2) (‘somewhat hard’) at 80 minutes ( p , 0.0001) (Table 3). 4.

Discussion

This study observed the EMG manifestations of localised muscle fatigue in the investigated shoulder muscles in both young and older groups during an 80-minute repetitive manual task. The EMG measures obtained during both dynamic task and isometric tests suggested shoulder muscle fatigue development (increased RMS and decreased MPF). The EMG manifestation of muscle fatigue coincided with increased subjective ratings of exertion. Age-modified EMG changes over time only for the trapezius and deltoid EMG measures during dynamic task, while no age effect was observed for EMG measures during isometric contractions and RPE. During the task, the decrease of EMG MPF in the older group was more monotonic than the young group for the upper trapezius and deltoid muscles. RRT of the upper trapezius muscle in the older group was greater than the young group throughout the task. However, there was no age-related difference in RRT temporal patterns. There were age-related differences in EMG manifestation of muscle fatigue development as indicated by the interaction between age and sessions. EMG power frequency in the older group showed a more monotonic decrease in the older group

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Figure 2. EMG RMS magnitude (normalised to RVE) and MDF during task. Values were averaged across 10 participants in each age group, and the error bars represent standard error of the mean. Post hoc pairwise comparison of session means within each age group were indicated by the letters above the plots such that A . B . C . D. Upper and lower case letters represent results of older and younger groups, respectively. Significant difference ( p # 0.05) of pairwise comparisons of session means between age groups was indicated by an asterisk underneath the bracket.

Ergonomics Table 2.

Statistical results of main effect and interaction for EMG variables measured during dynamic task. Age F

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EMG amplitude as RVE (%) Upper trapezius 3.21 Anterior deltoid 0.20 Middle deltoid 0.30 Posterior deltoid 1.45 Infraspinatus 0.33 MPF Upper trapezius 4.50 Anterior deltoid 1.29 Middle deltoid 2.76 Posterior deltoid 2.92 Infraspinatus 0.11 RRT Upper trapezius 3.93 Anterior deltoid 0.00 Middle deltoid 0.02 Posterior deltoid 0.67 Infraspinatus 0.38

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Age £ session

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0.074 0.652 0.586 0.229 0.567

48.02 23.76 17.78 19.06 23.76

< 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001

10.10 7.89 14.40 18.84 0.53

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69.72 30.87 35.35 25.26 7.26

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5.84 6.40 11.65 12.22 1.74

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2.44 1.04 1.06 3.42 0.73

0.034 0.391 0.380 0.005 0.604

1.11 0.74 1.84 0.82 0.73

0.345 0.527 0.139 0.485 0.534

Note: Bold values indicate p , 0.05.

compared with the younger group. However, age-related differences were less consistent in EMG RMS amplitude. For the middle deltoid, EMG amplitude increased rapidly in the first 40 minutes and remained unchanged (across sessions) in the last 40 minutes in both age groups. In addition, the increase in the older group was greater than the young in the first 20 minute. This might due to different motor learning processes between age groups, which is beyond the scope of this study and warrants further research in the future. Potential muscle adaptation process may be a possible explanation of the plateau of EMG amplitude in the latter half of the task. Similar patterns were observed for the posterior deltoid except that older group showed increase of EMG amplitude also in the last 20-minute session. The upper trapezius muscle EMG amplitude increased over four sessions probably because of the constant loading in this muscle during the task. The observed EMG manifestations of trapezius muscle fatigue are in line with the trend of fatigue development previously reported for lowintensity repetitive tasks (Bosch, De Looze, and Van Dieen 2007; Bosch et al. 2009). Muscle fatigue development in different muscles is likely to be associated with their functions in shoulder motion. The task speed in the current task was designed so that the arm had to perform reaching motions continuously. Therefore, the weight of the upper extremity due to gravity needed to be supported constantly, creating sustained static loading in the upper trapezius muscle in addition to dynamic loading resulting from the repetitive reaching motion. The loading in the deltoid muscle, in addition, was mainly affected by the dynamic loading associated with the kinematics of the upper arm. Active adjustment of the kinematics to combat muscle fatigue may be a potential explanation of the non-linear trend observed in deltoid muscle EMG amplitude. For example, decreased shoulder adduction/abduction torque and increased shoulder kinematic variability over time were observed. The details of upper extremity kinematics are beyond the scope of this article and described elsewhere (Qin et al. 2014). As a rotator cuff, infraspinatus helps laterally rotate humerus. Therefore, repetitive external rotation of the shoulder joint was likely a reason of EMG manifestation of infraspinatus fatigue. The older group showed less RRT during the task than the young group in the upper trapezius muscle. The Cinderella hypothesis postulates that long-term low-level workloads with low degree of muscle rest can cause selective over-usage of low-threshold muscle fibres, leading to WMSDs (Ha¨gg 1991). Based on this theory, it has been hypothesised that a low degree of muscle rest is a risk factor for the development of WMSDs. Studies have shown that a low amount of short muscle rest periods (gaps) predicted future development of WMSDs (Veiersted, Westgaard, and Andersen 1993) and among ˚ stro¨m 1997; Sandsjo¨ et al. 2000). Lower rest time in this workers with self-reported shoulder/neck complaints (Ha¨gg and A study suggests that older workers may have higher risk of developing upper trapezius symptoms than younger workers. In addition, the increase of deltoid muscle RRT over time may suggest that participants were able to adjust loading patterns of this muscle in a way that might reduce the risks. The observed patterns in RRT were consistent with changes in other EMG measures. For instance, sustained low RRT in the trapezius muscle coincided with increased RMS and decreased MPF across sessions; increased RRT in the deltoid muscle coincided with RMS (both groups) and MPF (in the young) plateau in the latter half of the task. An age difference was only observed in the upper trapezius RRT likely because of higher relative

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Time (min)

15

Young 10

Old

5 0 0

10

20

30

40

50

60

70

80

Time (min)

Figure 3. EMG RRT (the duration of rest as a percentage of total time) during task. Values were averaged across 10 participants in each age group, and the error bars represent standard error of the mean. When significant session effect was found, post hoc pairwise comparison of session means were indicated by the letters above the plots such that A . B . C. Younger and older groups were pooled for post hoc analyses because no age £ session interaction was found.

muscle loading in this muscle than the other muscles selected. This is due to the fact that the upper trapezius muscle plays a major role in lifting the shoulder joint, and sustained both static and dynamic loading during the task as discussed earlier. Although there is consistent evidence in the literature suggesting that older adults are more resistant to muscle fatigue than young adults during isometric muscle contractions (e.g. Hunter, Critchlow, and Enoka 2005; Yassierli et al. 2007), a few studies have suggested that this age-related advantage was lost during dynamic contractions (e.g. Yoon, SchlinderDelap, and Hunter 2013); some even suggested reversed age effect during dynamic contractions (Avin and Law 2011; Christie, Snook, and Kent-Braun 2011). Our results provided evidence supporting the conclusion that older female adults were not more fatigue resistant than young adults during the simulated low-intensity dynamic tasks in this study, at least in the selected shoulder muscles. Furthermore, some muscle EMG parameters indicated that older adults tended to develop muscle fatigue faster, and may potentially have higher risk of musculoskeletal symptoms in the trapezius muscle. Age-related changes in neuromuscular system (e.g. decrease in muscle strength and slower contractile properties) may be possible mechanisms of interaction between age and contraction mode on muscle fatigue (Kent-Braun 2009). The load of the current task was mainly due to the weight of the upper extremity. The estimated upper extremity masses were similar between age groups (Table 1); however, performing this functional task may result in higher relative intensity for the older adults than the young because of loss of strength with age.

Ergonomics Table 3.

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Isometric test results and RPE presented as mean (SD). Effect p-values 20 min

40 min

60 min

80 min

Age

Time

Age £ time

100 (0) 100 (0) 100 (0) 100 (0) 100 (0)

120 (19) 122 (18) 122 (23) 121 (28) 111 (19)

133 (20) 125 (17) 128 (23) 124 (29) 119 (27)

135 (29) 127 (24) 122 (28) 122 (36) 125 (26)

142 (34) 132 (29) 135 (32) 130 (39) 131 (30)

0.418 0.256 0.829 0.596 0.638

< 0.0001 < 0.0001 < 0.0001 0.001 < 0.0001

0.291 0.651 0.747 0.703 0.571

58.4 (7.1) 59.7 (6.6) 57.4 (5.8) 56.5 (7.6) 50.0 (12.2) 0 (0)

57.8 (6.5) 59.9 (8.4) 57.1 (8.2) 56.0 (7.4) 50.0 (12.2) 2.3 (1.5)

57.3 (6.5) 56.6 (7.1) 53.6 (7.6) 55.0 (8.0) 48.8 (11.22) 2.9 (1.6)

57.3 (6.7) 56.7 (7.9) 55.2 (8.5) 55.3 (7.9) 48.0 (11.4) 3.7 (1.8)

56.1 (7.1) 54.0 (8.6) 52.9 (8.5) 54.1 (8.1) 48.0 (11.3) 4.0 (2.2)

0.097 0.771 0.067 0.439 0.234 0.528

0.028 < 0.0001 < 0.0001 0.278 0.146 < .0001

0.914 0.525 0.599 0.815 0.171 0.730

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Baseline RVE (%) Upper trapezius Anterior deltoid Middle deltoid Posterior deltoid Infraspinatus MPF (Hz) Upper trapezius Anterior deltoid Middle deltoid Posterior deltoid Infraspinatus RPE

Note: Bold values indicate p , 0.05.

Age modification effect was found during dynamic but not isometric contractions in this study. One explanation could be the loss of type II fibres and fast motor units with ageing (Lexell, Taylor, and Sjostrom 1988; Lindstrom et al. 1997). In general, the composition of type II (fast-twitch) muscle fibres is about 30% in the descending part of the trapezius muscle (Lindman, Eriksson, and Thornell 1991) and 50% in the deltoid and infraspinatus muscles (Srinivasan et al. 2007). The agerelated changes in muscle fibre composition may negatively affect the fatigability among older participants during dynamic contractions, particularly at a fast speed. Avin and Law (2011) suggested that the slowing of contraction and relaxation times that occurs with ageing may cause a downward shift in the force – velocity curve, and therefore lead to a decreased ability to maintain power (force £ velocity) over time. This age-related change in muscle property would impair power generation during dynamic contractions but not during isometric contractions. Callahan and Kent-Braun (2011) tested the hypothesis that age-related shifts in the force – velocity relationship impact the fatigue response in a velocity-dependent manner. The results indicated that older group fatigued more than the young group during dynamic contractions performed at a relatively high angular velocity, while the opposite was true during isometric contractions. In addition to decreases in musculoskeletal function due to the development of age-related degenerative disorders, loss of tissue strength with age may increase the probability or severity of soft tissue damage from a given load. This may partly explain why median days away from work due to injuries and illnesses increase substantially with age (BLS 2006). The observed age-related difference of upper trapezius muscle response in the older group is especially alarming because this region is a prevalent site of work-related pain (Veiersted and Westgaard 1993). Older workers are likely to be subject to higher risk of trapezius muscle pain and other symptoms than younger workers when exposed to prolonged repetitive work. Rantanen and colleagues (Rantanen et al. 1999) argue that the interaction effect of strength decline caused by fatigue and ageing is an important contributor to disability in the elderly. Workplace accommodation for the older workers is, thus, necessary to reduce their risks of shoulder disorders. Task design temporal interventions such as job variation and rotation, more frequent breaks, and intermittent rather than continuous tasks can be effective prevention strategies. In addition, there is abundant evidence in the literature showing that both young and older adults could benefit from strength/resistance training in terms of increased strength, endurance, neuromuscular adaptation, as well as improved resistance to fatigue (e.g. Costill et al. 1979; Hainaut and Duchateau 1989; Van Roie et al. 2013; Walker et al. 2013). Further knowledge about the effect of such interventions on biomechanical loading could help work planning to reduce the risks of WMSDs for both young and older workers. Subjective ratings of exertion in the shoulder region increased over time in both age groups, which was consistent with EMG measures. Side-by-side comparison between RPE and EMG amplitude measured during isometric tests showed close resemblance, particularly to the upper trapezius muscle. There was no age effect on RPE and EMG measures during isometric tests. Besides the age-related differences discussed earlier, this lack of power to detect the interaction between age and repetition might be explained by limited samples. However, more samples interrupt the task and may introduce additional unwanted muscle fatigue. The findings here may be limited to the study design and experimental conditions tested. Age-related differences in muscle fatigue may be specific to the task and study population, and task characteristics, such as contraction type, intensity, speed, duty cycle and index of fatigue used, may influence study outcomes (Christie, Snook, and Kent-Braun 2011). The

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task in this study was highly repetitive at a controlled speed but relatively low in intensity. Changes made to these task parameters may lead to fatigue development patterns that are different from those observed in this study. For example, different work– rest schedules are likely to affect the slopes of temporal changes because breaks allow recovery from muscle fatigue. In addition, the duration of the simulated task was 80 minutes, one-sixth of a typical 8-hour working day. Direct extrapolation of the results to a real work environment is difficult because muscle fatigue development can be affected by many variables mentioned earlier. Bosch, De Looze, and Van Dieen (2007) showed indications of muscle fatigue development during light manual work over the course of an 8-hour working day. Individual factors other than age (e.g. gender, with/without pain) could also affect the outcome (Coˆte´ 2012). This study recruited only female participants; whether the conclusions can be applied to the male population needs additional evaluation. The study had limitations in that participants only used their dominant hand during the task. Although the prevalence and severity of WMSDs as well as the biomechanical loading are usually higher in the dominant hand (Qin et al. 2008), future study is warranted to understand the load sharing between both arms in two-handed work common in the industry. The sample size may limit the statistical power to detect the differences between two age groups. However, significant group differences that were observed in some of the dependent variables suggested sufficient power for those parameters. Future research examining other muscles (e.g. biceps and triceps brachii) could provide additional information on muscle co-contraction and force distribution. Although commonly used in ergonomic research, signals measured using bipolar EMG electrodes are limited in the spatial distribution of the potentials. Multi-electrode arrays (one- or twodimensional) can provide additional information from estimates of muscle fibre conduction velocities (Merletti, Farina , and Gazzoni 2003), and may have more values in studying muscle fatigue-related changes in EMG signals. Future research also needs to investigate the association between muscle fatigue development and muscular pain and injury outcomes in young and older adults.

Acknowledgements The authors thank Dr Nils Fallentin and Dr Yulan Liang for their comments and suggestions to improve this manuscript, and Jacob Banks, Niall O’Brien and Amanda Rivard for their assistance in data collection.

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